Caveats for Meshing Gears of Dissimilar Pressure Angle?

Vex sells two CIM motor pinions with 11 and 13 teeth that use the center distance for 12 and 14 teeth pinions, respectively. I’d tried to avoid them in the past because I was convinced Vex was using some dark form of witchcraft to make these gears work, but now that I have some familiarity with the concept of addendum modification and profile shifting, I can confirm that Vex uses witchcraft. But it’s good witchcraft, so these profile shifted pinions should actually be superior to the non profile-shifted versions.

However, this should theoretically raise issues when mating the profile-shifted pinions with every other gear Vex sells. Looking at the CAD models and drawings of Vex’s standard gears, it doesn’t look like they have any sort of negative profile shifting to match these two specific pinions. This effectively means that the profile-shifted pinions have a slightly larger pressure angle than the standard gear they’re mating with. Normally, meshing gears of dissimilar pressure angles is a big no-no, but this is so slight that it should be fine.

That being said, are there any caveats or limitations to using gears with slightly different pressure angles or profile-shifted pinions? From what I understand, these pinions will cause more interference and wear when being driven (as opposed to driving) than the standard profile gears. Is that all, or are there additional considerations for using profile-shifted pinions?

tl;dr: Can somebody please get me Buckingham’s Manual of Gear Design?

What literature are you reading where you’re seeing slightly larger pressure angles? I’m not super familiar with mating shifted gear profiles and non shifted profiles so not sure whether thats the case or not.

Whenever you use shifted profiles, the manufactured pressure angle and the working pressure angle diverge. They are still labeled and sold by their manufactured pressure angle, but since the base diameter has changed, the pressure angle will change as well.

Welp, as I’ve typed this, I realize I may have gotten my signs crossed. At first I just went off the fact that positive profile shifting causes the profile to resemble larger pressure angles, what with the wider tooth thickness and narrower top land. But now that I think about it, this might have been incorrect. I know the working pressure angle changes, but I can’t remember whether the change is positive or negative. I’ll try and give you a definitive answer on this within the weekend, but right now it’s almost 5 in the morning and I’m supposed to be studying for finals. Or sleeping. That too.

EDIT - I forgot one of my better sources.

Interestingly enough, this source only adds to my confusion. I’ll sleep on it, and get back to you.

I think if you compare base pitch you’ll see if the gears will work together.

Are you referring to the diametral pitch?

No, base pitch. The pitch diameter and pressure angle are sort of random numbers. You can rate them anywhere you want. Standard gears have standard tooth proportions. If the gears aren’t standard, the pitch at the base diameter needs to be the same for both gears (IIRC.)

Both the normal gear, and these profile shifted pinion gears have involute tooth profiles. In use the pressure angle (and thus the net force vector) will be different than the standard 20 degrees.

The change in pressure angle alters the efficiency somewhat; as the Tangental force turning the gear is a function of the cosine of the pressure angle.

Buckingham’s text is 50 years old. Shigley’s Machine Design book has been the standard ME text book for thirty years at just about all major US universities.

The fatigue life of these gears might be lower, but it would never be an issue during a single FRC season or two.